Mid-Autumn Festival is a holiday to be with and remember family during the night that has the “largest, fullest” moon of the year. (According to the Lunar Calendar, this is the month when the moon is the closest to the Earth, which is what makes it the “larger, fullest” moon of the year).
The original mooncakes were made in a round shape to represent the full moon. Tradition has it that you should spend the night of this holiday with your family and eat mooncake, to represent family closeness/togetherness – as in a family circle.
Other Loungers, please add to this if you have more information. My understanding is strictly as a layman.